Electrical machines are widely used in industrial applications. More than 80% of rotating machines worldwide are induction machines, and they are responsible for consuming around 50% of the total power generated in industrialized nations.
Induction machines are also used in safety and mission critical applications such as actuators in aircraft and propulsion motors in marine vessels.
It is therefore important to improve the reliability, and hence availability for function, of induction machines, e.g. motors, by early and reliable detection of faults. Electrical stress and thermal stress are the primary reasons for faults in induction machines. If these faults are undetected at early stages then these faults can lead to electrical or mechanical failure, which can cause permanent damage to the machine, and potential excessive loss of revenue.
The Broken Rotor Bar (BRB) fault is one of the predominant failure modes of induction machines, e.g. motors. The consequences of this fault include excessive vibrations, poor starting performances, torque fluctuation, and high thermal stress. If this fault remains undetected it may lead to potentially catastrophic failures. Thus, it is important to detect this particular fault to prevent permanent failure of induction machine.
The prior art discusses ways to detect this particular fault. In general, a typical methodology for detecting this fault is motor current signature analysis (MCSA). This method utilizes the machine stator current frequency signatures to detect the BRB fault. Under normal operation of a three phase induction motor, the three phase flux generates forward rotating magnetic field which rotates at synchronous speed with the rotor. A rotor fault such as a BRB fault introduces asymmetry in the machine current, which produces a backward rotating field, rotating at a slip frequency of the machine.
The interaction of this backward field with the stator windings induces an EMF at a frequency of (1−2s)fs, where fs is the supply frequency (or fundamental line frequency), and s is the slip of the machine. Because of cyclic current variation, the rotor speed oscillation induces upper sidebands of frequencies (1+2s)fs. Hence the frequency of the BRB spectral components in the stator current can be expressed as,fbrb=(1±2ks)fs  (1)
Where, fbrb is the BRB frequency, fs is the supply frequency, s is the slip and k=1, 2, 3, 4 . . . which denotes the index of the sidebands around the supply frequency fs.
In general, a sideband of any selected frequency is a pair of frequency components, respectively being higher and lower in frequency than the selected frequency by the same difference (in frequency). As per equation (1), the frequency component of interest is the fundamental, or supply, frequency (fs). The fundamental frequency is the supply frequency of the input current/voltage to the motor.
This classical method for detecting BRB faults is based on identifying and extracting the twice slip frequency sidebands around the fundamental (or base) supply frequency (also known as the fundamental line frequency), as represented by equation (1). For illustration purposes, FIG. 1 and FIG. 2 respectively show the line current frequency spectrum of a healthy induction machine (e.g. a motor) and an induction machine experiencing a BRB fault. The sidebands of the fundamental supply frequency as circled in FIG. 2 are characteristic of a BRB fault.
Alternatively to using the classical equation above, the prior art suggests other ways to detect faults in induction machines, in particular way to detect BRB faults.
For example, US20100301792 discloses a method for detecting an anomaly in an induction machine, and discloses two different embodiments. In the first embodiment, the instantaneous impedance is used to detect a BRB fault, the instantaneous impedance is calculated from the sequential voltages and currents. The instantaneous impedance under healthy and BRB fault are shown in FIG. 9 and FIG. 10 respectively in US20100301792. As shown therein in FIG. 10, the low frequency modulation in impedance is used to diagnose the BRB fault.
In a second embodiment, a volt-ampere method is proposed to reduce the computational burden of division by introducing multiplicative approach for coherent modulation and for the pronounced effect of fault signature. The transformation is utilized to compute the instantaneous alpha and beta component of the voltages and currents. Using these voltage and current components four different instantaneous volt-amperes are calculated by combining different combination of currents and voltages. The low frequency component in the instantaneous of power is the signature of a BRB fault as shown in FIG. 12B of US20100301792. The flow chart summarizing the approach is shown in FIG. 25 of US20100301792.
Another alternative prior art disclosure is provided in EP2113780B1. This prior art discloses a method for detecting damage caused to the short-circuit rings and/or bars of asynchronous motors. According to the disclosure, broken bars produce a significant alteration in the length spanned by each pole over the air gap of the machine and said alteration constitutes the parameter that is used as fault detection. The variations in the length of the magnetic poles are detected by measuring the flux linkage at one of the stator teeth using an auxiliary winding/search coil which is disposed in said stator tooth. The measured magnetic flux linkage is used to determine the value of EMF induced in the auxiliary coil situated around stator tooth, and then the period of EMF is determined by calculating time between the successive zero crossing. The period of EMF is compared with the period corresponding to the power supply frequency (f) of the motor, and interpreting as an indicator of damaged rotor bars and/or short-circuit rings if the period of EMF does not coincide with period corresponding to supply frequency (f).
Another prior art disclosure is provided by US8405339B2, in which is proposed a system and method for detecting a rotor fault condition in an AC induction machine which includes BRB faults and bearing faults. The system includes a processor programmed to receive voltage and current data from an AC induction machine, to generate a current frequency spectrum from the current data, and to identify rotor-fault related harmonics in the current frequency spectrum. The processor is also programmed to calculate a fault severity indicator using the voltage and current data, identify fault related harmonics, and motor specifications. The processor then generates an alert based on the possibility of rotor fault.
Yet another prior art disclosure is provided by U.S. Pat. No. 5,049,815, in which is disclosed a method for detecting rotor faults in an induction motor by analysis of a frequency spectrum of the current drawn by the motor under test. A method and apparatus is disclosed for detecting rotor faults in induction motors which relies solely on passive monitoring and analysis of the motor current. Signals indicative of the current drawn by the motor are digitized, stored, and digitally processed using a Fast Fourier Transform (FFT) to generate a frequency spectrum of the motor current. According to the disclosure, when rotor faults exist, signal peaks should appear in the motor current at frequencies calculated according to the following equation:
      f    k    =                                          f            0                    ⁡                      (                                                            k                  p                                ⁢                                  (                                      1                    -                    s                                    )                                            ±              s                        )                          ⁢                                  [                  prior          ⁢                                          ⁢          art          ⁢                                          ⁢          equation                ]            ⁢                          ⁢              f        k              =                  f        0            ⁡              (                                            k              p                        ⁢                          (                              1                -                s                            )                                ±          s                )            Where, fk is the broken rotor bar fault frequency, p is the motor pole-pairs, f0 is the supply frequency (also referred to as the fundamental line frequency), s is the per unit slip of the motor or slip frequency/f0 and k=1, 2, 3, etc. According to the disclosure, the fundamental motor current frequency is identified and, based on an estimation of the motor slip frequency (typically obtained from the motor nameplate data), a search is conducted for current peaks in excess of an established threshold in a sideband of the fundamental frequency over a search range predicated on the slip frequency estimate. If no current peaks are found, the rotor is declared to be fault-free. However, if current peaks are noted in the fundamental frequency sideband(s), then the frequency of each is declared a slip frequency candidate, and checks are conducted in appropriate harmonic sidebands, at frequencies predicted on the basis of each slip frequency candidate, for matching current peaks. According to the disclosure only certain harmonics are investigated—in particular, only those harmonics at k/p=1, 5, 7, 11, 13 etc. As long as matching current peaks are noted, the slip frequency candidates remain qualified. The search is extended to higher harmonic sidebands until all but one of the slip frequency candidates are disqualified. The one remaining candidate is then declared the slip frequency of the motor and is utilized in a rotor fault analysis of the motor current spectrum to determine the nature and severity of the fault.
However, these prior art methods have significant disadvantages. For example they:                a. Require high frequency resolution of current signals        
The frequency of the BRB fault signature (which is a function of slip) is close to the fundamental supply frequency, which makes it difficult to detect unless the frequency spectrum of the measured current has a high resolution. The high frequency resolution requires more data points, hence it is computationally intensive and may slow down the decision making process. In particular, U.S. Pat. No. 5,049,815 discloses to begin the process of identifying a rotor bar fault on the basis of a search conducted for current peaks in excess of an established threshold in a sideband of the fundamental frequency, and to declare that the rotor is fault free if no current peaks are found. Thus, U.S. Pat. No. 5,049,815 requires very high resolution data processing.                b. Require high slip to detect fault        
The detection of twice slip frequency under no load condition is not possible in the cited prior art since the current in the rotor bar is negligible. Hence, the prior art typically overloads the machine to increase the currents in the rotor bars to detect the fault (for example, the slip is increased to 35% more than the full load slip). Increase in slip for constant load can also be achieved by reducing the input voltage to the motor which in turn will reduce the torque generated and thereby reduce the rotor speed. Hence the slip of the motor can be increased even under no load conditions.                c. Are unable to detect low severity/incipient fault        
The magnitude of the BRB fault frequency components under low severity condition are relatively small compared with the magnitude of the fundamental supply frequency, which will eventually lead to elimination of the components as noise by fault detection and diagnosis system. Again, U.S. Pat. No. 5,049,815 discloses to begin the process of identifying a rotor bar fault on the basis of a search conducted for current peaks in excess of an established threshold in a sideband of the fundamental frequency, and to declare that the rotor is fault free if no current peaks are found. Therefore, there is a significant risk when following the teaching of U.S. Pat. No. 5,049,815 that a genuine fault may not be reliably identified.